Fixation Drift Increases as a Function of Time-on-Task

Ocular fixations contain microsaccades, drift and tremor. We report an increase in the slope of linear fixation drift as a function of time-on-task (TOT). We employed a very large dataset (322 distinct subjects, multiple visits per subject). Subjects performed a random saccade task. The task, in which the target dot jumped randomly over the display area every 1 sec, was 100 sec in duration. Fixations were identified using a published classification method. For each fixation, we regressed eye position against time across multiple segment lengths (50, 100, 200, 300, 400, and 500 ms). We started with the first sample and continued until no further regressions were possible based on the particular segment length being evaluated. For each segment length, each fixation was characterized by a single value: the maximum slope over the segment length. The slopes were expressed in deg/sec. We were not interested in the direction of the linear drift so we took the absolute value of the slope as the measure. For data analysis, each 100 sec task was divided into five 20 sec epochs. We found that median slope increased across epochs in both session recordings. Although similar trends were found regardless of segment length, the results were clearer and more consistent when using segment lengths of 200 ms or greater. Although we describe these changes in linear drift as related to time-on-task (TOT), we think it is likely, though no certain, that these effects are due to some sort of short-term oculomotor fatigue.